1. Field of the Invention
The present invention relates to a frame synchronizing signal insertion system.
2. Description of the Prior Art
In a transmission system for time-division multiplexed digital signals, frame synchronizing signals are added, at a transmitting end, to a plurality of digital pulses for conversion into a multiplex signal with a predetermined frame pattern so as to achieve frame synchronization between multiplex conversion sections at the transmitting and receiving ends. In practice, the multiplex conversion section at the transmitting end comprises a plurality of multiplexers which are separately arranged in casings or cabinets. Pulses from these multiplexers are time-division multiplexed to obtain a single multiplex signal which is then transmitted to the receiving end through a channel.
Conventionally, a frame synchronizing signal is added to the pulses in one of the plurality of multiplexers at the transmitting end. For this reason, when any multiplexer other than the multiplexer which is subject to frame synchronizing signal insertion operates erroneously or stops operating accidentally, such a defective operation cannot be detected by a frame sync detector at the receiving end. As a result, the receiving end cannot transmit an alarm signal or the like to indicate malfunctioning of the multiplexer at the transmitting end.